Technical Field
The present invention relates to photovoltaic devices and methods, and more particularly to devices and fabrication methods employing deposition temperature grading.
Description of the Related Art
Solar cells employ photovoltaic cells to generate current flow. Photons in sunlight hit a solar cell or panel and are absorbed by semiconducting materials, such as silicon. Carriers gain energy allowing them to flow through the material to produce electricity. When a photon hits a piece of silicon, the photon may be transmitted through the silicon, the photon can reflect off the surface, or the photon can be absorbed by the silicon, if the photon energy is higher than the silicon band gap value. This generates an electron-hole pair and sometimes heat, depending on the band structure.
When a photon is absorbed, its energy is given to a carrier in semiconductors. Electrons in the valence band may be excited into the conduction band, where they are free to move within the semiconductor. The bond that the electron(s) were a part of form a hole. These holes can move through the semiconductor creating mobile electron-hole pairs.
A photon need only have greater energy than that of a band gap to excite an electron from the valence band into the conduction band. Since solar radiation is composed of photons with energies greater than the band gap of silicon, the higher energy photons will be absorbed by the solar cell, with some of the energy (above the band gap) being turned into heat rather than into usable electrical energy.
A solar cell may be described in terms of a fill factor (FF). FF is a ratio of the maximum power point (Pm) divided by open circuit voltage (Voc) and short circuit current (Jsc):
  FF  =                    P        m                              V          oc                ⁢                  J          sc                      .  The fill factor is directly affected by the values of a cell's series and shunt resistance. Increasing the shunt resistance (Rsh) and decreasing the series resistance (Rs) will lead to a higher fill factor, thus resulting in greater efficiency, and pushing the cells output power closer towards its theoretical maximum.
Light induced degradation occurs in an amorphous semiconductor structure when the structure becomes saturated by incoming radiation (light soaked). The structure begins to degrade due in part to the reconfiguration of hydrogen atoms, which results in passivation loss and bond breaking between constituent materials. This degradation process becomes prominent if there exists a band offset at a p-i interface and/or at a transparent conductive oxide (TCO) to p+ interface. The Staebler-Wronski effect (SW effect) also affects solar devices and needs to be minimized.